Understanding white-tailed deer social behavior is critical if we are to predict and prevent the spread of chronic wasting disease (CWD). In this regard, I question if deer management agencies have adequately considered this issue before formulating and implementing CWD control measures for free-ranging whitetails.
Deer can contract CWD through contact with contaminated vegetation, soil or water. However, transmission of the disease from one animal to the next is believed to be more common in free-ranging deer. Incubation of the disease appears to be highly variable, but in some cases might take five or more years.
Research conducted under the direction of Edward Hoover, at Colorado State University, has shown that saliva from an infected animal alone can transmit CWD. This means, of course, that social habits such as touching noses and licking to groom each other can effectively transmit CWD.
Hence, CWD can be considered a social disease, as it would most likely be transferred among animals that are socially compatible, and can be spread considerable distances via dispersal of infected animals.
Researchers recognize that different social structures and behaviors, often the result of specific deer population management practices, or differing environments, can minimize the spread of CWD, but in some cases may actually trigger infected animal dispersal and enhance CWD spread.
I’ve discussed whitetail dispersal in a number of Deer & Deer Hunting articles. My purpose here is to revisit certain findings provided in those earlier articles and look at current thinking relative to the potential for female whitetails to spread social diseases such as CWD.
Social Organization
As with mule deer, red deer, elk, moose and other ungulates, the white-tailed deer sexes live separately during much of the year. In a natural situation (i.e., unhunted), related female whitetails live in a matriarchal society, composed of mothers, daughters, grandmothers, great-grandmothers and so forth. Members of these close-knit kinship groups normally show strong attachment to their ancestral range, tend to establish home ranges closely aligned with those related does, and seasonally band together, sometimes forming large compatible groups and plenty of opportunities to transmit CWD.
My research indicates that does fawning for their first time tend to establish fawning territories next to their mothers. This is probably adaptive behavior because the mother might help the young doe defend her fawns from predators. Does pregnant with their second litter are more inclined to disperse a quarter-mile or so to raise their fawns, thereby gradually increasing the size of the ancestral range — as well as the opportunity to spread social disease.
Adult bucks, on the other hand, form fraternal or bachelor groups generally composed of unrelated males. Yearling bucks are the chief dispersers, largely due to aggression and dominance by older female relatives. Although adult males tend to have a higher incidence of CWD than adult females, there is no evidence that males are inherently more susceptible to CWD than females.
In either case, however, the size and complexity of the social group, as well as dispersal tendencies, varies depending upon many factors (but especially population management and habitat characteristics) that influence the herd’s sex-age composition and density, as well as incidence and potential to spread CWD.
Is Localized Whitetail Management the Answer?
New York researcher William Porter proposed that whitetail populations could be effectively managed by targeting all individuals in a social group. Theoretically, because a group of related does controls and shares an ancestral range, eliminating such a group would leave that range vacant (and CWD-free) for an extended period. He proposed that removal of all related females from an area could result in low deer density in that area for 10 years or more.
Several studies have shown that such an approach might be feasible in certain areas — especially in urban-suburban settings. However, other researchers have questioned the widespread application of the approach because whitetail social organization varies regionally.
The Newborn
Most studies have shown low CWD infection rates for fawns that are 5 to 10 months of age. Even so, this suggests that some young fawns are probably infected prior to weaning and could even implicate CWD transmission from mother to fetus. However, to my knowledge, this has not been proven.
More likely, young fawns acquire the disease from their infected mothers as a result of intensive grooming during the early stages of fawn rearing and nursing. Otherwise, the fawn less than 4 to 6 weeks of age is isolated from other deer that are potential carriers of CWD, due to the mother’s aggressive behavior during early stages of fawn rearing.
The Social Female
When considering aspects of deer social behavior, remember one important feature: Young whitetails seek companionship and guidance, and maternal loss can have profound effects on their behavior — depending upon their immediate (i.e., back-up) social environment. If their mothers or other close female relatives are not available, they might disperse considerable distances to find friendly associates.
Typically, only 13% to 20% of yearling females disperse to new home ranges. However, in some areas, a comparatively high percentage of yearling females, and even doe fawns, disperse for a variety of reasons.
Studies conducted in Wisconsin, Minnesota, Illinois, Mississippi and Texas have shown that long-distance (50 to nearly 140 miles) dispersal of young female whitetails occurs most frequently in areas of low deer density or sparse forest cover, and sometimes is the result of intensive deer harvest that disrupts normal social behavior patterns.
Removing highly productive does makes good sense, of course, where deer numbers must be reduced to keep the herd in balance with a range’s carrying capacity. However, as noted years ago by Deer & Deer Hunting field editor, Bob Zaiglan: “There are potential drawbacks of prematurely removing does that have fawns.”
First Orphaning Study
One of the first studies on the effects of orphaning in white-tailed deer was conducted by Dennis Woodson and co-workers in a 3-square-mile Virginia enclosure. In 1973, researchers captured and marked 65 newborn fawns. Of 52 surviving to weaning age, 21 were orphaned in autumn when their mothers were captured and removed. They found a higher dispersal rate among orphans, 24%, compared to non-orphans, 6%, but dispersal distance was minimal due to confinement.
Farmland Doe Orphans
Dwayne Etter and other researchers from the Illinois Natural History Survey examined the dispersal behavior and survival of 13 female fawns accidentally or intentionally orphaned during winter and spring. They found that 69% of the orphan fawns dispersed from their birth range, compared to only a 37% dispersal rate for non-orphans.
Likewise, studies conducted by Charles Nixon and co-workers reported long-distance dispersal of doe fawns on intensively farmed land in central Illinois, where more than one-half of the doe fawns typically breed when they are 6 to 7 months old.
In their study, 36% of pregnant doe fawns and 23% of barren doe fawns dispersed an average of 30 miles when they were 10 to 12 months old.
In addition, 21% of the farmland does dispersed each spring when they were nearly 2 years old. The researchers speculate lack of successful breeding, because fawns might have been a factor stimulating these yearlings to disperse, because barren yearlings commonly revert to fawn behavior and seek their mother’s guidance when she tolerates other deer.
Clearly, the best evidence indicates that the amount of available forest cover relative to the proportion of young breeding does, was a major factor determining the likelihood of young doe dispersal in this intensively farmed region. Apparently, given the shortage of suitable fawn-rearing habitat, pregnant doe fawns and yearlings were forced to disperse considerable distances to find suitable fawn-rearing habitat. Hence, they generally settled in areas of very low deer density.
Socially Isolated Yearlings
Depending upon herd density and structure, yearling deer (males and females 12 to 16 months of age) are forced into a precarious (i.e., wandering) behavioral adjustment due to their mother’s sudden anti-social tendencies associated with raising newborn fawns. It’s more than coincidence that deer-vehicle accidents escalate during this time of year, as socially isolated yearlings scramble to find vacant range and compatible associates.
Likewise, it’s equally important to recognize that young deer in this age group seek companionship, sometimes forming distinct, temporary yearling groups comprised of both sexes — some of which might carry CWD and wander extensively. For example, one young doe (presumably orphaned) not only dispersed a straight-line distance of 61 miles out of a Wisconsin CWD area, but traveled roughly 287 miles during dispersal, exploratory and transient movements during a year.
Northern Yearling Females
Michael Nelson and Dave Mech, researchers with the U.S. Fish and Wildlife Service, were among the first to use telemetry equipment to carefully track the dispersal patterns of young does. Their study was conducted in heavily forested northern Minnesota, near the northern limit of the whitetail’s range. Deer in this area live at low densities (probably less than 5 per square-mile) tend to migrate from summer to winter range annually and endure heavy predation by gray wolves.
In the study, seven of 35 yearling does (20%) dispersed 11 to 104 miles from their birth range. The average distance was 36.5 miles. Five yearlings dispersed in late May at the onset of the fawning season. Two others dispersed in late June at the end of the fawning season.
In the Minnesota study, one of these does apparently dispersed with its brother. This suggests sibling and other social relationships (i.e., yearling groups) might influence the likelihood of yearling doe dispersal.
The Minnesota dispersing does did not travel in a straight line, but instead altered or reversed their direction when traveling. This was a lengthy process, involving 12 to 69 days. While dispersing, individuals used areas 28 to 147 times larger than normal adult home ranges.
In a separate study, the same researchers observed the behavior of radio-tagged twin fawns that had lost their mother after migrating to wintering cover. At break-up, the orphaned fawns left winter shelter, but wandered aimlessly, eventually returning to settle near where they had spent the winter — indicating that migratory fawns require guidance not only to find suitable wintering habitat, but must also be led back to their ancestral summer range as well.
Nelson and Mech could not explain why some young does dispersed such long distances or why they settled where they did. Since these young does were not pregnant, and were not crowded due to high deer density, they obviously weren’t looking for solitude to raise fawns.
Recent Thinking
An interesting test of the localized management hypothesis was conducted by Chris Comer and his cohorts from the University of Georgia. They examined genetic relationships and spacing of female deer on a 27 square mile Savannah River site located in South Carolina.
Based on DNA analysis of individuals and measured distance between radio-collared doe pairs, they found that many related does established home ranges farther apart than expected, while many unrelated does had overlapping home ranges.
Overall, their results indicated a low degree of genetic structuring, suggesting does on the Savannah River site were more inclined to disperse from their natal range, and less likely to form cohesive social groups.
One of the most recent studies conducted on female whitetail dispersal and disease spreading potential was by Marie Tosa and colleges from Southern Illinois University. They questioned the benefits of selectively removing female deer from social groups to decrease herd density with the intent of reducing the spread of social diseases such as CWD.
Specifically, they were interested in determining the social behavior of surviving female whitetails following family group disruption via selective sharpshooting. They did so by comparing the behavior of adult female and juvenile deer surviving family group disruption with others from intact family groups.
Over a four-year study, conducted in southern Illinois, researchers found that remnant adult does spent less time associating with neighboring groups. However, socially isolated juveniles demonstrated greater area coverage and joined intact neighboring family groups — hence increasing the potential to spread CWD despite lower deer density.
Tosa and her group conclude the following: “… our findings suggest that disease management should aim to remove entire social groups of deer instead of separate individuals, if feasible.”
Conclusions
Although seldom referred to, I initiated studies designed to evaluate the effects of whitetail social disruption via selective removal of individual deer nearly 50 years ago. Since then researchers have found that whitetail social patterns vary a great deal regionally, depending upon habitat conditions, reproductive potential and herd sex/age structure as determined by mortality factors.
Clearly, there are no cookbook herd management formulas for preventing the spread of CWD, and some practices might actually enhance disease spread. Certainly, at least in areas where female whitetails exhibit strict family group formation, attempted removal of entire social groups might prove rewarding, as long as no young infected deer (fawns or yearlings) are left on the ancestral range. Lone surviving adult females are more inclined to remain on ancestral range, and actually reduce their area coverage, whereas socially isolated young deer are more apt to disperse long distances and possibly spread disease in the process.
Therefore, given what we currently know about this intriguing subject, it’s my view that selective harvesting of young deer (male and female) should be given top priority, if we are to minimize deer dispersal and curb the spread of social diseases such as CWD.
— John Ozoga has been D&DH’s top research contributor for more than 20 years. He is a retired deer research biologist.
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LITERATURE CITED
Comer, C.E., et el. 2005. “Fine-scale Genetic Structure and Social Organization in Female White-tailed deer.” Journal of Wildlife Management 69:332-344.
Ozoga, J.J. 2000. “John Ozoga’s Whitetail Intrigue.” Krause Publications. Iola, WI. 206 pages.
Ozoga, J.J. 2002. “Whitetail Dispersal, Part 2.” Deer & Deer Hunting 25:(7):24-32.
Ozoga, J.J. 2014. “Whitetails on the Move Spread CWD.” Deer & Deer Hunting 38(5):63-71.
Tosa, M.L. et al. 2017. “Localized Removal Affects White-tailed Deer Space Use and Contacts.” Journal of Wildlife Management 81:26-37.
